Method for operating an internal combustion engine

ABSTRACT

In a process for operation of an internal combustion engine having a NOx storage catalytic converter, after desulfuration is required the mode of operation of the internal combustion engine is changed to λ 1 &lt;1 and secondary air is injected upstream from the NOx storage catalytic converter ( 4 ) during a first time interval T 1 . After secondary air injection is interrupted, the internal combustion engine is operated at λ 2 &lt;1 during a second time interval T 2 . Normal operation of the internal combustion engine is subsequently resumed. This process makes simple desulfuration of the NOx storage catalytic converter ( 4 ) possible (FIG.  1 ).

The invention relates to a process for operating an internal combustionengine as specified in the preamble of claim 1 and to a device forapplication of the process.

BACKGROUND OF THE INVENTION

In the case of internal combustion engines, lean-mix operated Ottoengines in particular, compliance with exhaust gas regulations requiresreduction of the nitrogen oxide (NOx) component. Either NOx storagecatalytic converters or DeNOx storage catalytic converters are used inthe exhaust system. NOx storage catalytic converters store the NOxpresent in exhaust gas and release it under certain operating conditions(λ<1). Desorption of the NOx on the surface of the NOx storage catalyticconverter at certain intervals is necessary, since the storage capacityof NOx storage catalytic converters is limited. In order to remainwithin the exhaust gas limits during NOx regeneration, in the downstreamcatalytic converter component the NOx released is reduced and thereduction agents HC and CO which are not completely converted areoxidized. A device such as this is disclosed in EP 560991.

One disadvantage of the state-of-the-art storage catalytic converters isthat their NOx storage capacity decreases over time as a result ofsulfur deposits, chiefly in sulfate form and the operating efficiencyand as a result the operating efficiency of the entire exhaust gassystem is significantly impaired. It is not possible to remain withinexhaust gas limits with “sulfur-contaminated” NOx storage catalyticconverters. Sulfur deposits are determined by the sulfur fraction offuel.

Desorption of sulfur from the surface of the NOx storage catalyticconverter is known to be theoretically possible. This process isdesignated as desulfurization in what follows. A prerequisite for theprocess is that the NOx storage catalytic converter be at a specifictemperature and that a reducing environment (sufficient HC and CO) bepresent at the same time.

However, these conditions can be fulfilled only after prolongedoperation of the internal combustion engine under full load (or highpartial load). Such an operating condition is vehicle-dependent ordriver-dependent and thus highly unpredictable. It depends on a largenumber of conditions such as driver intent, traffic and road conditions,etc.

Along with the point in time, the duration of this mode of operation andthus the period of operation available for desulfuration are of coursealso constantly unpredictable.

Desulfuration of a NOx storage catalytic converter on the basis offull-load or high partial-load operation is not possible for everyvehicle with the regularity required.

Hence it is possible that the operating efficiency of a NOx storagecatalytic converter may be greatly impaired by sulfur deposits and thatthe NOx component of exhaust gas will rise sharply as a result.

The type of desulfuration of a NOx storage catalytic converter describedabove presents a number of disadvantages. Such desulfuration isextremely irregular and can be carried out only under very specialoperating conditions (full load or high partial load) accompanied byincreased fuel consumption. While it is true that the condition of theNOx storage catalytic converter is monitored during operation, it is notpossible to interpret the results to determine the origin of increasedNOx emissions. In addition, a driver may be directed to drive under fullload, but only rarely can this be realized.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to develop a process of operating aninternal combustion engine which does not present the disadvantagesindicated in the foregoing, a process by means of which desulfuration ofa NOx storage catalytic converter is possible at all times, whichrequires no costly additional equipment for its application, and whichis simple and cost effective in application. Another object of theinvention is to develop a device for application of the process.

This object is attained by means of the features indicated in claims 1and 13.

The essential idea of the invention is that, with a requirement set fordesulfuration of the NOx storage catalytic converter, in an initial timeinterval T₁ operation of the internal combustion engine is set to avalue λ<1 and at the same time secondary air is blown in upstream fromthe NOx storage catalytic converter in order to heat this catalyticconverter. In a subsequent time interval T₂ the secondary air injectionupstream from the NOx storage catalytic converter is interrupted and theinternal combustion engine is operated at a value λ₂<1. Desorptionproper of the sulfur on the surface of the NOx storage catalyticconverter takes place during this second time interval T₂, since anadequate reducing atmosphere is now present.

The process corresponding to time intervals T₁ and T₂ may be repeatedseveral times in succession. Normal operation of the internal combustionengine is resumed when the desulfuration has been completed.

The essential advantage of the process claimed for the invention isrepresented by the circumstance that desulfuration is carried outautomatically, the necessary measures can be carried out over a broadrange of operating conditions of the internal combustion engine, and atthe same time the process has no noticeable effect on the performance ofthe internal combustion engine. The driver will not notice thedesulfuration process.

Advantageous developments of the invention are indicated in thesubsidiary claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in detail with reference to an exemplaryembodiment illustrated in the drawing, in which

FIG. 1 presents a diagram of an internal combustion engine with twothree-way catalytic converters and a NOx storage catalytic convertermounted between them,

FIG. 2 an internal combustion engine as shown in FIG. 1, with cooledconnecting lines, and

FIG. 3 an internal combustion engine as shown in FIG. 1 with a bypassline.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An internal combustion engine with an engine 1 and an exhaust gasassembly 20 is shown in diagram form in FIG. 1. The exhaust gas assembly20 consists of two three-way catalytic converters 3 and 5 between whicha NOx storage catalytic converter 4 is installed.

The three-way catalytic converter 3, which serves as starting catalyticconverter, is connected to the engine 1 directly by a manifold 2 a. Acontinuous oxygen sensor 10 a is mounted upstream from the three-waycatalytic converter 3. The three-way catalytic converter 3 may bedesigned optionally with or without an O₂ reservoir. It assumes thefunction of total conversion of pollutants until the NOx storagecatalytic converter 4 reaches its operating temperature. In the startupphase the engine 1 is operated preferably with λ=1 or periodically λ<1if necessary until the three-way catalytic converter 3 starts up.

The three-way catalytic converter 3 is connected to the NOx storagecatalytic converter 4 by an input pipe 2 b. A temperature sensor TS1 anda continuous oxygen sensor 10 b are mounted in each input pipe 2 b. TheNOx storage catalytic converter 4 is connected to the three-waycatalytic converter 5 by an input pipe 2 c. Another temperature sensorTS2 and a conventional oxygen sensor 10 c or optionally a NOx sensor 12are mounted in the input pipe 2 c.

A secondary air pump 22 with connecting lines 26 a, 26 b, 26 c blowssecondary air into the exhaust gas system 20. A selectable functionvalve 24 is used to determine the point of air injection, that is,whether secondary air is to be blown into the exhaust gas system 20upstream or downstream from the three-way catalytic converter 3 ordownstream from the NOx storage catalytic converter 4. The secondary airmass may also be controlled or regulated by way of the valve.

The secondary air pump 22 may be represented by an electric blower, amechanical compressor, or a turbocharger. Delivery of air from one ormore cylinders by disconnection of the fuel supply is also conceivable.

The secondary air mass may be determined with an air mass gauge (notshown), such as the air mass gauge for the engine air mass or a separateair mass gauge.

The secondary air mass may be regulated, for example, by means of thevalve 24 or by timed activation of the secondary air pump 22.

The direction of flow of the exhaust gas stream is indicated by arrows.

For the sake of clarity, only the secondary air intakes SL of thesecondary air system and the location of the temperature sensors areindicated in FIGS. 2 and 3.

The internal combustion engine shown in FIG. 2, in contrast to that inFIG. 1, has a water cooling unit 6 a or 6 b on the manifold 2 a or onthe connection 2 b between three-way catalytic converter 3 and NOxstorage catalytic converter 4.

The water cooling serves to some extent to protect against thermaldestruction of the NOx storage catalytic converter 4 in higher loadranges, since conventional storage materials exhibit extensive thermalaging at a maximum temperature of around 800° C.

In addition, the range of operation of the NOx storage catalyticconverter 4 can be extended by means of water cooling. Effective NOxstorage takes place only within a specific temperature range(approximately 200 to 450° C.). However, this maximum temperature isreached in lean-fuel operation even at a vehicle speed of about 70km/hr. NOx storage is possible at even higher speeds if the exhaust gasis cooled.

The corresponding cooling devices, including the pertinent control unit,are not shown in detail.

Only the manifold 2 a is provided with a water cooling unit 6 a in oneexemplary embodiment not shown in the drawing.

For considerations of space the NOx storage catalytic converter 4 andthe three-way catalytic converter 5 are mounted in a single housing.

The internal combustion engine shown in FIG. 3 has a bypass line 9 bymeans of which the exhaust gases circumvent the first three-waycatalytic converter 3 and the NOx storage catalytic converter 4 shown inFIG. 3 has a bypass line 9 with a connecting piece 9 a by means of whichthe first three-way catalytic converter 3 or the NOx storage catalyticconverter 4 are bridged. In this way the NOx storage catalytic converter4 can be protected against overheating. A bypass valve 8 is used tocontrol the exhaust gas flow.

In this exemplary embodiment the three-way catalytic converter 3 and theNOx storage catalytic converter 4 are mounted in a single housing.

The three-way catalytic converter 3 or 5 may also be in the form of aconical catalytic converter of metal and may be mounted in a commonhousing with the NOx storage catalytic converter 4 (not shown in thedrawing).

All sensors and all control mechanisms are connected to an enginecontrol unit 100. Sensor signals are interpreted in this engine controlunit 100 and the corresponding control commands are sent from this unitto the control mechanisms.

The operation of the invention is described in detail below.

During operation of the internal combustion engine the operatingcapacity of the NOx storage catalytic converter 4 slowly decreasesbecause of sulfur deposits. Consequently, the operating efficiency ofthe NOx storage catalytic converter 4 is monitored as described below.

The NOx emission of the engine 1 depends on the performancecharacteristics and may be determined by means of the engine controlunit 100. The current assigned NOx storage capacity of the NOx storagecatalytic converter 4 may also be determined on the basis of data storedin the engine control unit 100. If the readings of the NOx sensor 12vary from the assigned values, the engine control unit 100 ordersdesulfuration of the NOx storage catalytic converter 4 (process step a).

The operating efficiency of the NOx storage catalytic converter 4 mayoptionally be monitored by means of the two oxygen sensors 10 b and 10c. The oxygen sensor 10 c downstream from NOx storage catalyticconverter 4 indicates the O₂ concentration accurately even during NOxregeneration. The O₂ concentration process varies in NOx regeneration asa function of thermal aging and the extent of sulfur contamination. Inboth circumstances earlier “breakthroughs” of reduction agents HC and COand faster changes in the O₂ concentration processes occur during NOxregeneration.

Demand for desulfuration by the engine control unit on the basis of thisinformation is also possible.

Desulfuration is introduced by switching operation of the internalcombustion engine to λ₁<1 (preferably λ<<1, e.g., 0.7-0.9) accompaniedby injection of secondary air upstream from the NOx storage catalyticconverter 4 during a first time interval T₁ (process steps b and c). Forthis purpose the secondary air pump 22 and the valve 24 areappropriately activated. This measure results in heating (exothermalreaction) of the exhaust gas flow downstream from the three-waycatalytic converter 3. The exhaust gases thereby heated act to heat theNOx storage catalytic converter 4.

During the first time internal T₁ (process step c) care must be taken toensure that the temperature of the NOx storage catalytic converter 4remains within certain limits. On one side 800° C. should not beexceeded, while on the other the minimum desulfuration temperatureshould be distinctly higher than about 600° C.

During the first time interval T₁ the engine is controlled so that thetemperature value (TS1), that is, the exhaust gas temperature upstreamfrom the NOx storage catalytic converter 4, falls within the 750° C. to800° C. range. If the maximum value of 800° C. is reached, value λ₁ isincreased for a brief period, the advance angle is changed, or thesecondary air amount is varied by timed activation in order to preventfurther heating.

The exhaust gas temperature downstream from the NOx storage catalyticconverter 4 also rises during time interval T₁. It is a gauge of thetemperature in the interior of the NOx storage catalytic converter 4.Secondary air injection is interrupted as soon as the temperature valuesensed by temperature sensor TS2 exceeds the minimum desulfurationtemperature by a minimal value (about 100° C.).

Hence the first time interval T₁ continues until, for example, thetemperature sensor TS2 senses the temperature value 700° C.

Interruption of secondary air injection upstream from the NOx storagecatalytic converter 4 at the proper time protects the latter fromoverheating.

The exhaust gas temperature upstream and downstream from the NOx storagecatalytic converter 4 can be controlled by use of two temperaturesensors TS1 and TS2. The heat absorption and at the same time thetemperature of the NOx storage catalytic converter 4 may be determinedby comparison of the two temperature values TS1 and TS2. As analternative, a temperature model may also be employed to determine thesevalues upstream from the NOx storage catalytic converter 4 and tomonitor the heating process appropriately, only one temperature sensorbeing used.

It is certainly expedient to adjust the total lambda value obtained bymeans of the secondary air injection upstream from or in the NOx storagecatalytic converter 4 during secondary air injection upstream from theNOx storage catalytic converter 4, that is, during time interval T₁.This total lambda value should be greater than 1 and should preferablyfall within the 1.05 to 1.2 range. This total lambda value may beaffected either by engine operation or by way of the secondary air massitself. In this process the engine control unit 100 controls thesecondary air mass so that this total lambda value is obtained or thesecondary air mass is controlled as a function of engine operation sothat this total lambda value is also obtained.

The secondary air mass can be regulated by control of the valve 24 ortimed activation of the secondary air pump 22.

The total lambda value is monitored by means of the continuous oxygensensor 10 b, which is mounted at the point of secondary air injection.

Secondary air injection during time interval T₁ results in oxygenenrichment (λ>1) in the exhaust gas unit 20 downstream from the point ofinjection. However, desulfuration is not possible in an oxygen-richatmosphere. Consequently, injection of secondary air is interruptedupstream from the NOx storage catalytic converter 4 in a second timeinterval T₂, the internal combustion engine then operating at λ₂<1(preferably 0.9<λ₂<0.99) (process steps d and e). This results in areducing atmosphere in the NOx storage catalytic converter 4 which makesdesulfuration possible. Since secondary air injection does not takeplace during time interval T₂, the temperature of the NOx storagecatalytic converter 4 drops to a point at which desorption of the sulfuris no longer possible for temperature reasons. If the temperature valueof the temperature sensor TS2 drops below the minimum desulfurationtemperature, process step 3 is interrupted and time interval T₂consequently ends.

If the requirement for desulfuration is set, “definite” sulfurcontamination is present. Breakdown of these definite sulfur depositsrequires a specific time interval which is a function of the temperatureof the NOx storage catalytic converter 4 and the air ratio λ upstreamfrom or in the NOx storage catalytic converter 4. This ratio correspondsto the supply of reducing agents (HC and CO). The amount of both isknown or is continuously measured. Hence, for example, a minimum timefor desulfuration as a function of exhaust gas temperature upstream fromthe NOx storage catalytic converter 4 and the λ value upstream from theNOx storage catalytic converter 4 may be determined in the enginecontrol unit 100.

If the minimum desulfuration time is not reached after one-timeexecution of process steps b to e, after process step e has beencompleted process steps b to e may be repeated (once or a number oftimes) until normal operation of the internal combustion engine is againsensed (process step f).

Switching to λ₁, λ₂ or normal operations is executed by the enginecontrol unit 100 so that no torque discontinuities or other changes invehicle performance occur which might be noticed by the driver.

In another exemplary embodiment secondary air injection between the NOxstorage catalytic converter 4 and a downstream three-way catalyticconverter 5 takes place during the second time interval T₂ (λ₂<1) forexhaust gas improvement during sulfur desorption. Valve 24 is activatedas required for this purpose.

Consequently, secondary air injection is not interrupted after atemperature value of 700° C. is reached at temperature sensor TS2 butrather is continued by switching of valve 24 downstream from the NOxstorage catalytic converter 4 and upstream from the three-way catalyticconverter 5. As a result, the engine may be operated at λ<1 (preferably0.9<λ≦0.99) and at the same time λ=1 can be achieved upstream from thethree-way catalytic converter 5. The advantage in this instance isefficient conversion of pollutants HC and CO in the three-way catalyticconverter 5 on the basis of the total lambda value λ=1 accompanied byrich-fuel operation of the engine for production of the reducing agentsnecessary for desulfuration of the NOx storage catalytic converter 4.Adjustment to λ=1 can be effected by timing the secondary air by meansof a timing valve or in measurement of the amount of fuel by means ofthe engine control unit. The latter operates only if the secondary airmass is more or less constant and thus the operating conditionsconstant. This procedure is, on the other hand, faster for the purposeand more efficient for exhaust gas conversion.

The essential advantage of the process claimed for the invention is thatit is very simple to carry out desulfuration of the NOx storagecatalytic converter 4.

The object of the invention is also a device for application of theprocess as specified in the following claims.

What is claimed is:
 1. A method of operating an internal combustionengine having a NOx storage catalytic converter, including: a) settingthe requirement of desulfurization of the NOx storage catalyticconverter; b) operating the internal combustion engine at λ₁<1 andinjecting secondary air upstream from the NOx storage catalyticconverter during a first time period; c) operating the internalcombustion engine at λ₁<1, where λ₂ is larger than λ₁, and discontinuingthe injection of secondary air during a second time period following thefirst time period; d) resuming normal operation of the internalcombustion engine.
 2. A method as specified in claim 1, includingrepeating steps (b) and (c) after completion of method step (c).
 3. Amethod as specified in claim 1, including operating the internalcombustion engine during step (b) so that the temperature value of theupstream temperature sensor does not exceed a maximum value.
 4. A methodas specified in claim 1, including using lambda values of 0.7<λ₁<0.89and 0.9<λ₂<0.99.
 5. A method as specified in claim 1, includingadjusting the injection of secondary air flow and operating the internalcombustion engine to create a total lambda value (λ_(T)) greater than 1,preferably 1.05<λ_(T)<1.2, prevailing upstream from the NOx storagecatalytic converter during method step (b).
 6. A method as specified inclaim 1, including monitoring the NOx storage catalytic converter forsulfur deposits by means of a NOx sensor.
 7. A method as specified inclaim 1, including monitoring the NOx storage catalytic converter forsulfur pollutants using a continuous oxygen sensor mounted upstream anda conventional oxygen sensor mounted downstream from the NOx storagecatalytic converter.
 8. A method as claimed in claim 1, includingswitching off the secondary air injection upstream from the NOx storagecatalytic converter as soon as a temperature value of an upstreamtemperature sensor exceeds a minimum desulfurization temperature plus aminimum value.
 9. A method as specified in claim 1, includingdiscontinuing the operation of the internal combustion engine at λ₂<1during the second time period as soon as a temperature value of adownstream temperature sensor falls below a minimum desulfurizationtemperature.
 10. A method as specified in one of claims 8 or 9,including assigning the minimum desulfurization temperature equal to600° C. and assigning the minimum value equal to 100° C.
 11. A method asspecified in claim 1, including injecting secondary air during step (b)downstream from the NOx storage catalytic converter but upstream from athree-way catalytic converter located downstream from the NOx storagecatalytic converter.
 12. A method as specified in claim 11, includingadjusting the injection of the secondary air flow and operating theinternal combustion engine to create a total lambda value (λ_(T)) equalto 1 (λ_(T)=1) upstream from the three-way catalytic converter.
 13. Ainternal combustion engine system comprising: a) an internal combustionengine having an exhaust system; b) a NOx storage catalytic converter ina path of the exhaust system; c) air injection means connected to theexhaust system for injecting air into an exhaust stream passing throughthe exhaust system; and d) control means for controlling the operationof the internal combustion engine and the air injection means; such thatat a desulfurization requirement, for a first time interval the controloperates the internal combustion engine at λ₁<1 and operates the airinjection means to inject air into the exhaust stream, and for a secondtime interval the control operates the internal combustion engine at aλ₂<1 where λ₂>λ₁ and discontinues the air injection means from injectingair.